So I want to know what's our policy on questions of negative mass and such.
First, I would reconsider your assumption that the question was closed purely because it asked about negative mass. The only way to know for sure is if those who voted to close decide comment on your question (which can no longer happen since you deleted it) or decide to post here. Typically, questions here are not closed purely because of their topic. Conversely, just because other negative mass questions were not closed does not mean that any question you post about negative mass will not be closed. Many other factors are considered.
With that being said, I will say that I was one of the users who voted to close that question. I did not do this because it was about negative mass. I voted to close for multiple reasons.
- The question was somewhat of a "What if" question. As in "What if negative mass existed, then could we do this thing?"
- The question was somewhat of a "check my reasoning here". Direct quote: "Now considering the figure I concluded that yes electricity can be generated perpetually. - Is it possible to do so?"
- The question was also fairly broad. Continuing the previous quote: "If not is there any other method?" This is a very broad question.
So summarizing, you posted a question that was "What if this was real? If it was, is my reasoning correct? If my reasoning is not correct, then how could you do this?" This, to me, is a fairly broad, hypothetical question, and this is why I thought the question should be closed.
General comments (not addressing OP's specific question):